Lipid Biosynthesis Dr. Fayez Almabhouh Assistant Professor, Biology and Biotechnology Department
Introduction • Lipids play a variety of cellular roles. • They are the principal form of stored energy in most organisms and major constituents of cellular membranes.
Introduction Specialized lipids serve as : • Pigments(retinal, carotene) • Cofactors (vitamin K) • Detergents (bile salts) • Transporters (dolichols) • Hormones (vitamin D derivatives, sex hormones) • Extracellular and intracellular messengers (eicosanoids, phosphatidylinositol derivatives) • Anchors for membrane proteins (covalently attached fatty acids, and phosphatidylinositol).
Introduction • The ability to synthesize a variety of lipids is essential to all organisms. • Like other biosynthetic pathways, these reaction sequences are endergonic and reductive. • They use ATP as a source of metabolic energy and a reduced electron carrier (usually NADPH) as reductant.
Biosynthesis of Fatty Acids • Fatty acid oxidation takes place by the oxidative removal of successive two-carbon (acetyl-CoA) units, biochemists thought the biosynthesis of fatty acids might proceed by a simple reversal of the same enzymatic steps.
However, as they were to find out, fatty acid biosynthesis and breakdown occur by different pathways, are catalyzed by different sets of enzymes, and take place in different parts of the cell. • Moreover, biosynthesis requires the participation of a three-carbon intermediate, Malonyl-CoA, that is not involved in fatty acid breakdown.
Malonyl-CoA Is Formed from Acetyl-CoAand Bicarbonate • The formation of malonyl-CoA from acetyl-CoA is an irreversible process, catalyzed by acetyl-CoA carboxylase. • The enzyme contains a biotin prosthetic group covalently bound in amide linkage to the ε-amino group of a Lys residue in one of the three polypeptides or domains of the enzyme molecule.
Formation of malonyl-CoA • The two-step reaction catalyzed by this enzyme is very similar to other biotin-dependent carboxylation reactions, such as those catalyzed by pyruvate carboxylase and propionyl-CoA carboxylase.
The carboxyl group, derived from bicarbonate(HCO3-), is first transferred to biotin in an ATP dependent reaction.
The biotinyl group serves as a temporary carrier of CO2, transferring it to acetyl-CoAin the second step to yield malonyl-CoA
Fatty acid synthesis: proceeds in a repeating reaction sequence. • The long carbon chains of fatty acids are assembled in a repeating four-step sequence.
The Fatty Acid Synthase ComplexHas Seven Different Active Sites • The core of the E. coli fatty acid synthase system consists of seven separate polypeptides (Table 21–1), and at least three others act at some stage of the process. • The proteins act together to catalyze the formation of fatty acids from acetyl-CoA and malonyl-CoA.
Before the condensation reactions that build up the fatty acid chain can begin, the two thiolgroups on the enzyme complex must be charged with the correct acyl groups.
Fatty Acid Synthase Receives the Acetyl and Malonyl Groups • First, the acetyl group of acetyl- CoA is transferred to the Cys -SH group of the β-ketoacyl-ACP synthase (KS). • This reaction is catalyzed by acetyl-CoA–ACP transacetylase (AT).
The second reaction, transfer of the malonyl group from malonyl-CoA to the -SH group of ACP, is catalyzed by malonyl-CoA–ACP transferase (MT), also part of the complex.
A four-step sequence: Step 1. Condensation: The first reaction in the formation of a fatty acid chain is condensation of the activated acetyland malonylgroups to form acetoacetyl-ACP,an acetoacetylgroup bound to ACP through the phosphopantetheine -SH group; simultaneously, a molecule of CO2is produced.
In this reaction, catalyzed by β-ketoacyl-ACP synthase (KS), the acetyl group is transferred from the Cys -SH group of the enzyme to the malonyl group on the -SH of ACP, becoming the methyl-terminal two-carbon unit of the new acetoacetyl group. The carbon atom of the CO2formed in this reaction is the same carbon originally introduced into malonyl- CoA from HCO3by the acetyl-CoA carboxylase reaction . Thus CO2 is only transiently in covalent linkage during fatty acid biosynthesis; it is removed as each two-carbon unit is added.
Step 2. Reduction of the Carbonyl Group • The acetoacetyl- ACP formed in the condensation step now undergoes reduction of the carbonyl group at C-3 to form D-β-hydroxybutyryl-ACP. • This reaction is catalyzed by β - ketoacyl-ACP reductase (KR) and the electron donor is NADPH.
Step 3. Dehydration • The elements of water are now removed from C-2 and C-3 of D-β-hydroxybutyryl-ACP to yield a double bond in the product, trans-Δ2- butenoyl-ACP. The enzyme that catalyzes this dehydration is β -hydroxyacyl-ACP dehydratase (HD).
Step 4. Reduction of the Double Bond • Finally, the double bond of trans-Δ2-butenoyl-ACP is reduced (saturated) to form butyryl-ACP by the action of enoyl-ACP reductase (ER); again, NADPH is the electron donor.
A saturated acylgroup produced by this set of reactions becomes the substrate for subsequent condensation with an activated malonyl group.
Translocation of butyryl group to Cys on β-ketoacyl-ACP synthase (KS) The enzyme that catalyzes this translocation is acetyl-coA-ACP acetytransacetylase (AT)
Beginning of the second round of the fatty acid synthesis cycle. • The butyryl group is on the Cys -SH group of KS. • The incoming malonyl group is first attached to the phosphopantetheine -SH group of ACP.
Then, in the condensation step, the entire butyryl group on the Cys -SH of KS is exchanged for the carboxyl group of the malonyl residue, which is lost as CO2 (green). This step is analogous to step 1.
The product, a six-carbon β-ketoacyl group, now contains four carbons derived from malonyl-CoA and two derived from the acetyl-CoA that started the reaction. • The β-ketoacyl group then undergoes steps 2 through 4
With each passage through the cycle, the fatty acyl chain is extended by two carbons. • When the chain length reaches 16 carbons, the product (palmitate, 16:0) leaves the cycle.
The Fatty Acid Synthase Reactions Are Repeated to Form Palmitate The overall process of palmitate synthesis. The fatty acyl chain grows by two-carbon units donated by activated malonate, with loss of CO2 at each step. The initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. After each two-carbon addition, reductions convert the growing chain to a saturated fatty acid of four, then six, then eight carbons, and so on. The final product is palmitate (16:0).
Throughout the process, the intermediates remain covalently attached as thioesters to one of two thiol groups of the synthase complex. • One point of attachment is the -SH group of a Cys residue in one of the seven synthase proteins (β-ketoacyl-ACP synthase); the other is the -SH group of acyl carrier protein.
Fatty Acid Synthesis Occurs in the Cytosol of Many Organisms but in the Chloroplasts of Plants • In most higher eukaryotes, the fatty acid synthase complex is found exclusively in the cytosol, as are the biosynthetic enzymes for nucleotides, amino acids, and glucose.
Long-Chain Saturated Fatty Acids Are Synthesized from Palmitate • Palmitate, the principal product of the fatty acid synthase system in animal cells, is the precursor of other long-chain fatty acids. • It may be lengthened to form stearate (18:0) or even longer saturated fatty acids by further additions of acetyl groups, through the action of fatty acid elongation systems present in the smooth endoplasmic reticulumand in mitochondria
Routes of synthesis of other fatty acids. Palmitate is the precursor of stearate and longer-chain saturated fatty acids, as well as the monounsaturated acids palmitoleate and oleate.
Biosynthesis of Triacylglycerols • Most of the fatty acids synthesized or ingested by an organism have one of two fates: • Incorporation into triacylglycerolsfor the storage of metabolic energy or • Incorporation into the phospholipid components of membranes.
The partitioning between these alternative fates depends on the organism’s current needs. • During rapid growth, synthesis of new membranes requires the production of membrane phospholipids; when an organism has a plentiful food supply but is not actively growing, it shunts most of its fatty acids into storage fats. • Both pathways begin at the same point: the formation of fatty acyl esters of glycerol.
Animals can synthesize and store large quantities of triacylglycerols, to be used later as fuel. • Humans can store only a few hundred grams of glycogen in liver and muscle, barely enough to supply the body’s energy needs for 12 hours.
In contrast, the total amount of stored triacylglycerol in a70-kg man of average build is about 15 kg, enough to support basal energy needs for as long as 12 weeks.
Triacylglycerols have the highest energy content of all stored nutrients—more than 38 kJ/g. • Whenever carbohydrateis ingested in excess of the organism’s capacity to store glycogen, the excess is converted to triacylglycerolsand stored in adipose tissue. • Plantsalso manufacture triacylglycerolsas an energy-rich fuel, mainly stored in fruits, nuts, and seeds.
Triacylglycerols and Glycerophospholipids are synthesized from the same precursors • In animal tissues, triacylglycerols and glycerophospholipidssuch as phosphatidylethanolamine share two precursors (fatty acyl–CoA and L-glycerol 3-phosphate) and several biosynthetic steps.
The vast majority of the glycerol 3-phosphate is derived from the glycolytic intermediate dihydroxyacetone phosphate (DHAP) by the action of the cytosolic NAD-linked glycerol 3-phosphate dehydrogenase; in liver and kidney, a small amount of glycerol 3-phosphate is also formed from glycerol by the action of glycerol kinase (Fig).
Glycerol 3-phosphate formation
The other precursors of triacylglycerolsare fatty acyl–CoAs, formed from fatty acids by acyl-CoA synthetases, the same enzymes responsible for the activation of fatty acids for β-oxidation
The first stage in the biosynthesis of triacylglycerolsis the acylation of the two free hydroxyl groups of L-glycerol 3-phosphate by two molecules of fatty acyl–CoA to yield diacylglycerol 3-phosphate, more commonly called phosphatidic acid or phosphatidate diacylglycerol 3-phosphate
Phosphatidic acid is present in only trace amounts in cells but is a central intermediate in lipid biosynthesis; it can be converted either to a triacylglycerol or to a glycerophospholipid.
In the pathway to triacylglycerols, phosphatidic acid is hydrolyzed by phosphatidic acid phosphatase to form a 1,2-diacylglycerol. • Diacylglycerolsare then converted to triacylglycerols by transesterification with a third fatty acyl–CoA.
Triacylglycerol biosynthesis in animals is regulated by hormones • The rate of triacylglycerol biosynthesis is profoundly altered by the action of several hormones. • Insulin, for example, promotes the conversion of carbohydrate to triacylglycerols. • People with severe diabetes mellitus, due to failure of insulinsecretion or action, not only are unable to use glucose properly but also fail to synthesize fatty acids from carbohydrates or amino acids.
Regulation of triacylglycerol synthesis by insulin. Insulin stimulates conversion of dietary carbohydratesand proteins to fat. • Individuals with diabetes mellitus lack insulin; in uncontrolled disease, this results in diminished fatty acid synthesis, and the acetyl-CoA arising from catabolism of carbohydrates and proteins is shunted instead to ketone body production and therefore lose weight.
An additional factor in the balance between biosynthesis and degradation of triacylglycerols is that approximately 75% of all fatty acids released by lipolysis are reesterified to form triacylglycerols rather than used for fuel. • This ratio persists even under starvation conditions, when energy metabolism is shunted from the use of carbohydrate to the oxidation of fatty acids.